To date these Puget Sound samples with a second, independent method, we analyzed individual tree-ring sequences for a jump in annual radiocarbon concentrations

To date these Puget Sound samples with a second, independent method, we analyzed individual tree-ring sequences for a jump in annual radiocarbon concentrations associated with an extreme solar proton event (30, 31). This rapid, large magnitude (~10‰) radiocarbon excursion between the years 774 and 775 CE is recorded globally in tree cellulose and can therefore be used as an exact geochronological anchor point (30, 32, 33) They found more exact dates because a massive solar storm in the past changed carbon isotopes worldwide, which is reflected in tree rings that grew during the storm. That's incredibly cool.